Complex Formation With Organic Acids

Organic acids are the other group of biomolecules that can function as chelators of heavy metals inside the cell, converting the metals to almost inactive and nontoxic forms. With regard to Al, at least two organic acids are known to function as chelators. One is citric acid [138]; nearly two-thirds of Al in hydrangea leaves remain present in the cell sap in soluble form as Al-citrate complex at a 1:1 molar ratio of Al to citrate, a nontoxic form of Al.

Another acid that has been reported to form intracellular complex with Al is oxalic acid [139]. About 90% of Al in buckwheat remains present as soluble oxalate Al complex in the symplasm, and the intracellular concentration of Al detected is as high as 2 mM. The complex occurs in molar ratio of 1:3, Al:oxalate. Oxalic acid can form three species of complexes with Al at an Al:oxalic acid molar ratio of 1:1, 1:2, and 1:3. The 1:3 Al-oxalate complex is the most stable, with a stability constant of 12.4 [140]. This stability constant is much higher than that of Al-citrate (8.1) or Al:ATP (10.9), meaning that formation of 1:3 Al-oxalate complex can prevent binding of Al to cellular components, thereby detoxifying Al very effectively. The report is in contrast to the order of stability constant for Al-organic acid complexes: Al-citrate > Al-oxalate > Al-malate [141]. It is not known, however, whether the Al complexes of citrate or oxalate remain located in cytoplasm or in the vacuole.

Among the heavy metals reported to be chelated by organic acids inside the cells are Zn and Ni. After exposure to high concentrations of various heavy metals, vacuoles of the Zn- and Ni-tolerant plants, as well as those of the nontolerant plants, often contain high concentrations of zinc and nickel [142,143], as well as some Cu and Pb [144] and Cd [145-147]. The results of the studies on Zn- and Ni-tolerant plants suggested that organic acids could be involved in their sequestration in the vacuole; the Zn-tolerant plants, including Silene vulgaris, exhibited enhanced accumulation of malate [148,149] and the Ni-tolerant plants showed accumulation of malate, malonate, or citrate [148,150] upon their exposure to Zn and Ni, respectively. The details of their transportation and sequestration inside the vacuole and the roles of the organic acids in the process are not available.

In one of the models for the transport of Zn into vacuole, it has been postulated that malic acid would bind Zn in the cytosol, thereby detoxifying it, and the Zn-malate complex would be transported over the tonoplast into the vacuole where it would dissociate [39]. After this, malate would be retransported into the cytosol. Vacuolar Zn would remain bound to stronger chelators, such as citrate, oxalate, etc., when present. Brune et al. [151] reported that barley mesophyll cell vacuoles contain appreciable concentration of phosphate (30 to >100 mol m-3); malate (>10 mol m-3); sulphate (>4 mol m-3); citrate (~1 mol m-3); and amino acids (>10 mol m-3) when grown in hydroponic culture. They hypothesized that these organic and inorganic salts interact with the divalent cations, thereby buffering the vacuolar free Zn concentration to low values even in the presence of high Zn levels (292 mmol m-3) in the vacuolar space.

According to Wang et al. [152], citrate is the most efficient ligand for metal complexation in the vacuole at vacuolar pH values of 6 to 6.5. The results of Brune et al. [151] demonstrate the importance of compartmentalization and transport as homeostatic mechanisms within leaves to handle possibly toxic zinc levels in shoots. The dependence of plants on organic acids for detoxification of Zn could be the reason for poor induction of PCs by the metal [74,76].

The mechanism of detoxification adapted by plant probably varies from metal to metal and, for a metal, from species to species, and it is difficult to reconcile the idea of tolerance by means of any single mechanism. For example, Zn-tolerant Agrostis capillaries and Silene vulgaris, which exhibit increases in malate levels [153], are only slightly Ni tolerant [43]; Ni-tolerant Alyssum bertolonii, which is very rich in malate [150], is nontolerant to Zn [39]. Similarly, as stated earlier, BSO increases the toxicity of Cd to the tobacco cells not selected for Cd tolerance, but not of Zn or Cu. Again, for Al detoxification, plants follow several strategies.

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